1. Field of the Invention
The present invention relates to a method for the reduction of formaldehyde emission in wood-base materials by treatment with polyamines. The present invention furthermore relates to a wood-base material which can be produced by the present method, and to laminates which can be produced by the present method and the use of these wood-base materials and laminates for the production of pieces of furniture, of packaging materials or in interior finishing.
2. Description of the Prior Art
Wood-base materials are an economical and resource-protecting alternative to solid wood and have a quite considerable importance in particular in furniture construction and as construction materials. Wood layers of different thickness, wood strips, wood particles or wood fibers of various timbers serve as starting materials. Such wood parts or wood particles are usually pressed with natural and/or synthetic binders and, if appropriate, with addition of further additives to give sheet-like or strand-like wood-base materials.
Binders used are frequently formaldehyde-containing adhesives, for example urea-formaldehyde resins or melamine-containing urea-formaldehyde resins. The resins are prepared by polycondensation of formaldehyde with urea or melamine. In order to obtain good adhesive properties, as a rule an excess of formaldehyde is used here. This may result in free formaldehyde being present in the finished wood-base material. Additional formaldehyde can be liberated by hydrolysis of the polycondensates. The free formaldehyde present in the wood-base material and the formaldehyde liberated by hydrolysis during the life of the wood-base material can be released into the environment.
Wood itself can also release formaldehyde into the environment, in particular after a heat treatment. Coated wood-base materials generally have a lower formaldehyde emission than uncoated substrates (“Holz als Roh-und Werkstoff” volume 47, 1989, page 227).
Above certain limits, formaldehyde can cause allergies or irritation of the skin, of the respiratory tract or of the eyes in humans. The reduction of the formaldehyde emission in components in the interior area is therefore an important concern.
A reduction of the formaldehyde emission by reduced addition of formaldehyde during the production leads to success only to a limited extent since the adhesive properties of the binder deteriorate with decreasing formaldehyde concentration and the setting of the adhesive is substantially slowed down. This leads to longer production cycles (described in “Holzwerkstoffe und Leime”, M. Dunky, P. Niemz, Springer Verlag Berlin-Heidelberg, 2002, 251-302).
A further possibility for the reduction of the formaldehyde emission consists in the addition of formaldehyde scavengers, such as urea, to the wood particles or to the formaldehyde resin. However, a disadvantage of this method is the slower setting rate of the resin. In addition, the mechanical properties of the products are adversely affected.
It has been possible in recent years to reduce the formaldehyde emission of the wood-base materials by various measures. Examples of these are:
G. Myers (Forest Products Journal 1986, Vol. 36 (6), 41-51) gives an overview of possible methods. These range from the use of low molecular weight formaldehyde scavengers, such as urea or ammonia, in solid form (for example as ammonium bicarbonate), in aqueous solution (for example urea solution) or in gas form (NH3) to the application of a coating which acts as a physical barrier.
The gassing of wood-base materials, in particular particle boards, with ammonia (RY AB method, Verko method) and the spraying of the particle boards with formaldehyde scavengers (Swedspan method) have become technically important (E. Roffael and H. Miertzsch, Adhasion 1990, 4, 13-19). In the Swedspan method (EP-B 0006486) the particle boards in the hot state are sprayed with aqueous urea solution or other solutions containing ammonia-eliminating substances. A disadvantage is the poorer coatability of the particle boards thus treated. In the gassing of the wood-base materials with ammonia (RY AB method, Verko method) that the formaldehyde emission increases again with increasing duration of storage has proven disadvantageous (page 16, E. Roffael and H. Miertzsch, Adhasion 1999, 4, 13-19).
WO 2004/085125 A2 describes a method for reducing the emission of adhesively bonded wood-base materials, in which mixtures of aldehyde- and isocyanate-reactive substances are applied to the straightened edges, lying perpendicular to the direction of adhesive bonding. Pollutants escaping both from the wood and from the adhesive are said to be retained thereby. Substances which have an amino or amido group, or hydroxyl-containing substances, can be used for this purpose. For example, urea, guanamine, ethylamine, ethanolamine, proteins, alcohols and carbohydrates are mentioned.
JP 2002-273145 A describes a method for reducing the formaldehyde emission of wood composites, in which a plurality of measures suitable in each case individually for reducing the formaldehyde emission are combined. The aqueous formaldehyde scavenger described is composed of from 20 to 50% by weight of urea, the remainder being a nonvolatile amine, an agent for increasing the permeability of the wood, so that the urea and the nonvolatile amine can penetrate into the composite, and a film-forming solid which, after drying, becomes a physical barrier for the formaldehyde on the wood composite. This film may adversely affect the coatability of the wood composites. Polyalkylenepolyamines are also described as nonvolatile amines. This class of substances is understood generally as meaning short-chain, linear polyamines of the formula H2N(—CH2—CH2—NH)n—H where n=2, 3 and 4.
In spite of many measures, such as the adaptation of the stoichiometry of the formaldehyde-containing binders or of the addition of formaldehyde scavengers, and different methods for the aftertreatment of wood-base materials, further measures are necessary in order to reduce the emission of formaldehyde from wood-base materials. Furthermore, the emission of formaldehyde from exposed areas which form after the final surface finishing and, if appropriate, an aftertreatment, for example by drilling, milling or sawing, have not been taken into account to date.
Such exposed areas are present, for example, as holes in shelf units or in the backs of pieces of furniture and are required for ensuring a flexibly adjustable height of the shelves. Furthermore, the edges of shelves which do not face the living area are generally not surface-coated and are therefore exposed. In the case of some shelves, even the complete underside is uncoated and therefore constitute exposed areas from which formaldehyde emerges.